htaccess rewrite for routing from a sub directory - .htaccess

Question

I've been using this .htaccess file for enabling the grav Cms for a few sites:
RewriteEngine On
RewriteCond %{REQUEST_URI} !^/grav/
RewriteRule ^$ /grav [QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /grav/$1 [L,QSA]
This works flawlessly as long the site is at the document root.
Now, I have a new site that starts in 2020/ and has Grav installed in 2020/grav.
I modified the above .htaccess to be:
RewriteEngine On
RewriteCond %{REQUEST_URI} !^/2020/grav/
RewriteRule ^2020$ /2020/grav [QSA]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^2020/(.*)$ /2020/grav/$1 [L,QSA]
This .htaccess file is located in /2020/.
While in the first case the url is left as is and Grav gets the rest of the url as parameters, in the second case (2020/) the url changes to /2020/grav, which is not what I want?
I wanted to check if it was a Grav issue and I've setup a simple test case, with the above .htaccess file and a grav/ directory with only an index.php file in there and I got the same result.
Any hint on how I have to modify the .htaccess file to get 2020/abc to stay as is in the url bar and at the same time have Grav to get the abc argument?

Answer 1

You can have this code in 2020/.htaccess:
RewriteEngine On
RewriteRule ^$ grav/ [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(?!grave/)(.+)$ grav/$1 [L,NC]
(?!grave/) is negative lookahead condition that will skip match if match already starts with grave/

Answer 2

The original question was about getting it to work with an .htaccess.
#anubhava gave a very good answer for the general case. But it sadly was not enough for Grav.
I got some more help in the Grav forums and at the end of a long journey Ole found out that the question has been asked in their forums a long time ago and a solution was found!
The /2020/.haccess can be simply defined as:
RewriteEngine On
RewriteCond %{REQUEST_URI} !^/2020/grav/
RewriteRule ^(.*)$ /2020/grav/$1
But what one need is to add a couple of configurations in the /2020/grav/user/config/system.yaml file:
custom_base_url: 'http://domain.tld/2020'
session:
path: /2020
You can find a more detailed question and Ole's reply in the Grav forums